The present invention is directed to a device for holding a sheet of material by means of a vacuum, in particular for holding films, foils, plates and the like.
A task of holding a sheet of material, such as films, foils and the like, on a drum rotating at high speed or, in other instances, on planer or curved plates, will occur in many fields of technology, particularly in the field of reproduction technology. When the holding device is provided with a plurality of suction bores, then a vacuum is suitable to provide a clamping means for holding the sheet of material onto the holding element or device.
A great variety of film formats occurs, however, in reproduction technology. Even film pieces having contours, which are completely irregular in shape, must often be clamped in the holding device. When those suction bores that are not covered by the film or sheet are simply left open, then extremely high powered vacuum pumps and high leakage losses through the uncovered bores must be accepted in order to also reliably hold a sheet which has a small, irregular shaped format.
Because considerable centrifugal forces oppose the retaining force in a drum scanner rotating at high speeds, this type of vacuum clamping of the films is not reliable enough especially for vacuum clamp cylinders.
German AS No. 1,164,224 discloses a vacuum clamp device, which has only specific zones of suction holes that can be activated on the basis of valves or cocks and appropriately laid out channels. This device, however, remains limited to a few standard formats and fails given originals having irregularly shaped contours. In addition, this device is very expensive to manufacture, especially when one wishes to employ them in the clamping cylinders. They also do not work automatically but must be manually pre-adjusted.
U.S. Pat. Nos. 3,307,816 and 3,307,817 each disclose vacuum clamp devices, wherein partly spring-loaded, spherical shaped or piston shaped valve bodies close suction bores that are not covered by the original. Also disclosed in the patents are porous inserts which are provided in the suction bores, which merely reduce the leakage rate by means of throttling the flow through the exposed bore.
Apart from the expensive and complicated manufacture as well as the risk of failure due to slight contamination, such elements, moreover, do not reliably close the valve spaces of the suction bores due to the high centrifugal forces in drum scanners rotating at high speeds. In such a case, the vacuum can easily collapse during the scanning procedure. With a collapsed vacuum, the original will fly off the rotating cylinder and can damage the apparatus or even be destroyed.
U.S. Pat. No. 2,694,337 discloses a suction plate which has a plurality of leaf spring valves that are automatically closed due to the air flow. These leaf spring valves are rigidly attached with screws at a defined distance from the bore to be closed and are attached in recesses of the suction plate which is complicated and hard to manufacture. For example, the leaf spring valves are not freely mobile. The manufacture of such suction plates is extremely involved and expensive.